Those links were originally put in, I imagine, because they were professional-type links. GitHub is a site that will let you showcase your work; Twitter, while it is a social network, is often also used by companies and independents to update people on company progress.
Instagram, to me at least, doesn't seem to fit with that ethos (which, by the way, I think is good - we're a professional community, not a social network, etc). The image that Instagram projects seems to be a very stereotypical social-network type of thing, sort of like Facebook (which I suppose is unsurprising, really). Given that it's a mismatch to the current ethos, that there's no option for a Facebook link either, and that you can link it in the free form text you get in your profile, I don't think this is necessary.